1. Field of the Invention
This invention relates to steel cords for the reinforcement of rubber articles such as pneumatic tires, industrial belts and the like and pneumatic radial tires using the same as a carcass ply for the improvement of tire durability.
2. Description of the Related Art
A typical example of rubber articles reinforced with steel cords, are known pneumatic tires. Among these tires, the tires for truck, bus and light truck use are common and have a carcass ply comprised of steel cords having a two or three layer twisting structure or so-called multilayer twisting structure.
In general, bending deformation is caused in the steel cord as a reinforcing material during the running of the tire, and hence fretting wear is created in each filament constituting the steel cord to inevitably decrease the sectional area of the filament and lower the tenacity of the steel cord.
If the decrease of sectional area in a portion of the filaments constituting the steel cord is violent, such a filament is apt to cause breakage to shock in tension or repetitive bending. Once the filament is broken, tensile stress increases in the remaining filaments to promote fatigue breakage of the steel cord. Therefore, in order to improve the durability of the steel cord, it is effective to avoid the premature breakage in a portion of the filaments and also it is desirable that the lowering of tenacity in all filaments constituting the steel cord is same.